This invention relates to the determination of the fissile fuel content of fuel for nuclear reactors. Such nuclear reactors are discussed for example in "Nuclear Power Engineering", M. M. El-Wakil, McGraw-Hill Book Company, Inc., 1962.
Nuclear reactors are typically refueled periodically with an excess of fuel sufficient to maintain operation throughout an operating cycle.
This excess of fuel results in an excess of reactivity which requires a control system of sufficient strength to maintain the effective multiplication factor at unity during reactor operation. The control system customarily comprises neutron absorbing or poison materials that serve to control the neutron population by nonfission absorption or capture of neutrons. Typically, the control system includes mechanical control in the form of a plurality of selectively actuatable poison containing control rods or the like which can be inserted into and withdrawn from the core as required.
Nuclear fuels include uranium and/or plutonium in suitable form. For example, in commonly used fuel for water cooled and moderated nuclear power reactors the fuel comprises uranium dioxide (UO.sub.2) in which from about 0.7 to 5.0 percent is fissile U-235 mixed with fertile U-238. Such nuclear fuel typically is in the form of sintered pellets contained in an elongated cladding tube to form a fuel element or rod as shown, for example, in U.S. Pat. No. 3,378,458. Such fuel elements are arranged in groups and supported in separately replaceable fuel assemblies in the core of the reactor. A typical fuel assembly is shown for example in U.S. Pat. No. 3,689,358.
To decrease local power peaking and to achieve desired power distribution, it is known to vary the fuel enrichment radially from element-to-element within the fuel assembly and axially along the lengths of the elements. That is, the fissile fuel content of a fuel element may be varied from zone-to-zone along its length.
It is also known to include in the fuel core a burnable poison such as gadolinium which is a strong neutron absorber but is converted by neutron absorption to an isotope of low control worth (neutron absorbing capacity). Such use of burnable poisons decreases the amount of mechanical control required and, by appropriate arrangement of the burnable poison, improvements in power distribution can be achieved.
Such burnable poisons frequently are incorporated in the fuel elements in a mixture with selected portions of the nuclear fuel. As in the case of fissile fuel content, the burnable poison content of a fuel element may be varied from zone-to-zone along its length. A zoned arrangement of burnable poison is shown, for example, in U.S. Pat. No. 3,799,839.
It is desirable for quality control and identification purposes during nuclear fuel handling and fuel element and fuel assembly manufacturing processes to provide rapid nondestructive methods for determining accurately the amount, enrichment and location of the fissile fuel along the length of a fuel element.
An early method for the nondestructive determination of the relative fissile concentration or enrichment of fuel material can be called "passive scanning". Such a method is based upon the quantitative detection of gamma rays of characteristic energy emitted during the natural radioactive decay of a fissile material such as U-235. Such a method is discussed by G. H. Morrison et al in an article entitled "Determination of Uranium-235 by Gamma Scintillation Spectrometry", in Analytical Chemistry, Vol. 29, No. 12, December 1957, pages 1770 and 1771.
Further development of this passive scanning method for enrichment measurement involved a correction of the U-235 indication based upon the detection of gamma rays emitted by daughter products of U-238. The method and systems employing such correction are discussed, for example, by C. N. Jackson, Jr., in an article entitled "Enrichment Tester for 0.15 to 3.0 Weight Per Cent U-235 Uranium Fuel" in Materials Evaluation, August 1966, pages 431-435, and by J. T. Russell in U.S. Pat. No. 3,389,254.
Since U-235 has a relatively long half-life its spontaneous disintegration rate and consequent gamma ray emission is low. Because of this and because of the statistical nature of the gamma ray emission, the scanning times required for accurate U-235 determination by the passive scanning method are undesirably long for production fuel use, particularly for low enrichment fuel material. For example, it was found that fuel elements in the order of 4.5 meters long containing fuel of 2 to 3 percent enrichment required a scanning time in the order of forty minutes where a single gamma ray detector was used to obtain suitable accuracy in enrichment measurement.
The throughput can be increased (that is, the scanning time for fuel elements can be reduced) by the use of a plurality of gamma ray detectors serially arranged adjacent to the fuel element being scanned. The gamma ray counts from the detectors attributable to each local segment of the fuel element are accumulated and summed, thus decreasing the scanning time required for a given accuracy in proportion to the square root of the number of detectors employed. An arrangement of a plurality of radiation detectors for sorting radiation emissive material is shown by F. T. Holmes in U.S. Pat. No. 2,717,693.
A method for the simultaneous nondestructive determination of the enrichment and fissile content of fuel material can be called "active scanning". In this method the fuel element is moved past a narrow beam of neutrons of selected energy and the gamma radiation from successive local portions of the element resulting from the neutron-induced fission of the fissile material, such as U-235, is detected. The radiation count from each local portion is a function of the intensity of the neutron activation beam and the concentration of U-235 in the local portion. Active fissile content analyzer systems have been described, for example, in German patent publication No. 1,923,983 (published Nov. 20, 1969) and by R. A. Pritchett in U.S. Pat. No. 3,018,374.
Active scanning systems have been placed in use in commercial nuclear fuel manufacturing facilities because of the accuracy of fissile content and enrichment determinations and the favorable scanning speed afforded thereby. However, in the case of fuel elements containing a burnable poison mixed with the fuel material, the neutron activation or active scanning method is not found effective. Such fuel elements may contain burnable poison, such as gadolinium, in amounts in the order of 1-10 weight percent. Such amounts of gadolinium render the fuel pellet or body substantially black to neutrons. That is, the activation neutrons are absorbed by the burnable poison at the surface of the body and do not penetrate sufficiently to provide adequate activation of the fissile material. Furthermore, the variability of the burnable poison concentration along the length of the fuel rod tends to mask the measurement of the fissile fuel content.
Present quality control techniques for the determination of enrichment and fissile content of gadolinium bearing fuel involve destructive measurement of presumably representative samples and are undesirable because they are not a direct measurement and they are expensive and labor intensive.
Thus there remains a need for nondestructive methods and means for determining rapidly, accurately and simultaneously the fissile content, enrichment and location of fuel material which may also contain amounts of burnable poison.
It is an object of the invention to provide a nondestructive method and apparatus for determining rapidly and accurately the fissile fuel content of a fuel element which also contains burnable poison. Another object is a passive scanning method wherein the fissile material content indication is corrected for fuel material density, thickness of the fuel cladding and burnable poison content in order to obtain greater accuracy.
Another object is the simultaneous determination of fissile content and enrichment of fuel material.
Another object is the simultaneous or separate determination of fissile content enrichment by location within a nuclear fuel element.